Microcavity plasmas, plasmas confined to a cavity with a characteristic spatial dimension <1 mm, have several distinct advantages over conventional, macroscopic discharges. For example, the small physical dimensions of microcavity plasma devices allow them to operate at gas or vapor pressures much higher than those accessible to a macroscopic discharge such as that produced in a fluorescent lamp. When the diameter of the microcavity of a cylindrical microplasma device is, for example, on the order of 200-300 μm or less, the device is capable of operating at pressures as high as atmospheric pressure and beyond. In contrast, standard fluorescent lamps operate at pressures typically less than 1% of atmospheric pressure. Also, microplasma devices may be operated with different discharge media (gases, vapors or combinations thereof) to yield emitted light in the visible, ultraviolet, and infrared portions of the spectrum. Another unique feature of microplasma devices, the large power deposition into the plasma (typically tens of kW/cm3 or more), is partially responsible for the efficient production of atoms and molecules that are well-known optical emitters. Consequently, because of the properties of microplasma devices, including the high pressure operation mentioned above and their electron and gas temperatures, microplasmas are efficient sources of optical radiation.
Research by the present inventors and colleagues at the University of Illinois has pioneered and advanced the state of microcavity plasma devices. This work has resulted in practical devices with one or more important features and structures. For example, semiconductor fabrication processes have been adopted to demonstrate densely packed arrays of microplasma devices exhibiting uniform emission characteristics. Arrays fabricated in silicon comprise as many as 250,000 microplasma devices in an active area of 25 cm2, each device in the array having an emitting aperture of typically 50 μm×50 μm.
The following U.S. patents and patent applications describe microcavity plasma devices resulting from these research efforts. Published Applications: 20050148270—Microdischarge devices and arrays; 20040160162—Microdischarge devices and arrays; 20040100194—Microdischarge photodetectors; 20030132693—Microdischarge devices and arrays having tapered microcavities; U.S. Pat. No. 6,867,548—Microdischarge devices and arrays; U.S. Pat. No. 6,828,730—Microdischarge photodetectors; U.S. Pat. No. 6,815,891—Method and apparatus for exciting a microdischarge; U.S. Pat. No. 6,695,664—Microdischarge devices and arrays; U.S. Pat. No. 6,563,257—Multilayer ceramic microdischarge device; U.S. Pat. No. 6,541,915—High pressure arc lamp assisted start up device and method; U.S. Pat. No. 6,194,833—Microdischarge lamp and array; U.S. Pat. No. 6,139,384—Microdischarge lamp formation process; and U.S. Pat. No. 6,016,027—Microdischarge lamp.
U.S. Pat. No. 6,541,915 discloses arrays of microcavity plasma devices in which the individual devices are mounted in an assembly that is machined from materials including ceramics. Metallic electrodes are exposed to the plasma medium which is generated within a microcavity and between the electrodes. U.S. Pat. No. 6,194,833 also discloses arrays of microcavity plasma devices, including arrays for which the substrate is ceramic and a silicon or metal film is formed on it. Electrodes formed at the tops and bottoms of cavities, as well as the silicon, ceramic (or glass) microcavities themselves, contact the plasma medium. U.S. Published Patent Application 2003/0230983 discloses microcavity plasmas produced in low temperature ceramic structures. The stacked ceramic layers are arranged and micromachined so as to form cavities and intervening conductor layers excite the plasma medium. U.S. Published Patent Application 2002/0036461 discloses hollow cathode discharge devices in which electrodes contact the plasma/discharge medium.
Additional exemplary microcavity plasma devices are disclosed in U.S. patent application Ser. No. 10/829,666, filed Apr. 22, 2004, entitled “Phase Locked Microdischarge Array and AC, RF, or Pulse Excited Microdischarge”; U.S. patent application Ser. No. 10/984,022, filed Nov. 8, 2004, entitled “Microplasma Devices Excited by Interdigitated Electrodes;” U.S. patent application Ser. No. 10/958,174, filed on Oct. 4, 2004, entitled “Microdischarge Devices with Encapsulated Electrodes,”; U.S. patent application Ser. No. 10/958,175, filed on Oct. 4, 2004, entitled “Metal/Dielectric Multilayer Microdischarge Devices and Arrays”; and U.S. patent application Ser. No. 11/042,228, entitled “AC-Excited Microcavity Discharge Device and Method.”
Producing plasmas with glow discharge in room air has been notoriously difficult but the ability to do so in a compact, low power device can have profound implications for, for example, environmental analysis of air samples. A direct current glow discharge has been reported. See, A.-A. H. Mohamed, R. Block, and K. H. Schoenbach, Direct Current Glow discharges in Atmospheric Air” IEEE Trans. Plasma Sci. 30, 182 (2002); Stark & Shoenbach; “Direct Current Glow Discharges in Atmospheric Air”. In those papers, it is reported that glow discharges were generated in atmospheric air by using a direct current diameter microcavities hollow cathode discharge as a plasma cathode. The microcavities ranged from 80 μm to 130 μm. Electrodes were molybdenum foils separated by dielectric. A third planar electrode extracts discharge glow. Glow discharges extending over distances up to 2 cm were produced, but doing so required a third electrode positively biased with respect to the microcavity cathode discharge. Sustaining voltages of the hollow cathode device ranged from 400-600 V, while the third planar electrode was maintained at 250V. See, Stark & Shoenbach, Appl. Phys. Lett., Vol. 74, No. 25 at 3770-71. Extracted discharge glows expanded well beyond the diameter of the discharge devices. The papers report that the bias voltage increases linearly with gap length. It was also observed that the size (cross-sectional area) of the plasma changed with distance from the microhollow cathode and its diameter increased to a value much larger than the transverse dimension of the microhollow cathode. The transverse spreading of the plasma was used to accomplish the merging two plasmas to form a homogeneous, large volume plasma.